This apparatus is directed to a type of radar system having an azimuthal directional antenna system to enable radar mapping of the geologic formations and their structures near a borehold in the earth. It is particularly useful in boreholes which are drilled through several formations where the interface between formations or interfaces therebetween and anomalies are of interest. One such device is shown by Shuck in U.S. Pat. No. 4,045,724. That is a device incorporating a highly directional transmitting antenna in the form of a horn. Other directional antennas suitable for use in a borehole include the structure shown in Gabillard which is U.S. Pat. No. 3,440,523 and Holser U.S. Pat. No. 3,412,815.
The present apparatus is particularly able to determine the presence and location of electromagnetic contrasts in the vicinity of a borehole drilled into geologic formations of the earth. The angular position of such anomolous contrasts is obtained by scanning the radar antenna so that its directional beam pattern is confined to a limited azimuthal zone oriented radially around the axis of the borehole. Distance to the anomaly is derived from knowledge of the electromagnetic wave propagration velocity in the drilled formation and the time delay required for the source energy to travel from the directional source antenna to the reflecting anomaly and return to an associated receiving antenna. If desired, the device can be stopped at a particular position in a borehole and scanned radially around the hole axis. It is desirable that the antenna have a relatively narrow beam. However, the beam should not be so narrow that the full circle of 360.degree. requires a large number of data points. This device sets forth an apparatus having a beam width wherein the beam is fairly definitive in the forward direction and the radiation pattern is substantially zero in the reverse direction, that being 180.degree. opposite from the front direction. At the 90.degree. position relative to the forward direction, beam strength is markedly reduced.
This enables the device to scan a full circle taking eight directional angular zones of view in a typical application. Closer spacing can be obtained by taking a data point every 30.degree. or so. The optimum seems to be about eight angular zones around the borehole axis. This beam provides sufficient definition in the return signals to enable the data to be meaningful.
One advantage of this angular scan spacing is that the device can be raised fairly rapidly in the wellbore. It utilizes an antenna system capable of forming a relatively narrow antenna pattern in a given direction.
This apparatus is able to rotate readily to scan in different azimuthal directions. The antenna structure is mechanically rotatable within its protective housed in a rotating housing. This housing and antenna construction is sized so that it fits neatly into holes drilled into earth materials such as hard rock, sedimentary rock, coal, salt, or other geologic formations. This type of construction permits the continuous rotation of the equipment to take data from different azimuthal directions. Moreover, it enables the antennas to be loaded for purposes of launching eletromagnetic wave radiation. Thus, the device incorporates a mechanism which rotates the antenna system for operation. The antenna fits snugly and neatly within a closed housing. The housing is filled with a liquid in typical applications to thereby increase the relative dielectric surrounding the antenna. This assists in changing the relative time relationship of the antenna elements. In the preferred and illustrated embodiment, the antenna is a pair of adjacent dipoles. The preferred form includes dipoles known as bow-tie dipoles. They are spaced so that a timed relationship in feeding exitation signals to them coupled with the spacing yields the angular selective antenna pattern described herein. Briefly, the dipoles are within a closed housing which is preferably filled with a liquid. Water is acceptable provided the ambient temperature range is not excessive. Water has a relative dielectric as high as 80. If low temperatures are encountered thereby running the risk of freezing of the liquid, the liquid can be composed of a mixture ethylene glycol and water. This will provide a relative dielectric constant of between 44 and 80 depending on the mix of the two liquids. It is also possible to use high dielectric solids such as barium titanate or other ferroelectric ceramics. Materials of this type provide a dielectric in the range of about 1,000 to 3,600. Such high dielectrics are not ordinarily necessary in light of the features, frequencies and spacing used in the device described below.
Through the use of this arrangement, the entire antenna system can be encapsulated within a diameter of three inches or less. This enables the sonde to be placed in a borehole as small as 43/4 inch diameter. Through the use of the time delay in the application of the single cycle pulses applied to the antennas (there being two separate dipoles), and through the further use of the housing filled with a dielectric, thereby changing the propagation velocity, the two antennas are excited individually with separate input signals but they radiate collectively in a fashion which Results in a pulse whose energy is concentrated in the forward direction with reduced strength on the sides and a substantially complete null to the rear.
A suitable operating frequency range of the directional antenna is between about 30 and 300 MHz. Within that range, it appears that the best frequency is approximately 100 MHz. Moreover, it has been determined that the best directional beam pattern is obtain by transmitting only one cycle from each of the two antenna that comprise the antenna system. This yields a pulse in the forward direction of the device, and a substantial null in the reverse direction. This makes the data obtained fairly directional.
With the foregoing in view, the system of the present disclosure incorporates an antenna system in a down hole sonde. The sonde is adapted to be lowered on a logging cable. The logging cable provides a means for transmitting power, control signals, and radar receiver output signals between surface equipment and down hole equipment.
The system further includes bow-tie dipoles, there being two with a particular arrangement between them. They are spaced in a particular fashion to control the phasing of the two radiated signals. The radiated signals are obtained by driving the two antenna dipoles with different timing of the separate antenna exitation signals. Further, the system incorporates a housing mounted on a rotor to enable the rotor to be rotated, thereby repositioning the composite antenna to scan in an azimuthed manner.